A hip joint comprises a socket or acetabulum and a femoral head or ball received in the acetabulum. Thus, the hip joint is a ball and socket joint which provides universal motion.
Various diseases, such as osteoarthritis, attack the hip joint and, when this occurs, it may be necessary to utilize an appropriate hip joint prosthesis to replace the femoral head and the acetabulum. This may also be necessary in other circumstances, such as in the case of certain hip joint fractures.
Deterioration of the acetabulum requires that an acetabular cup be mounted in the acetabulum to provide a socket for slidably receiving the prosthetic femoral head. If the medial wall of the acetabulum is sufficiently weakened, it may be necessary to protect this wall with a protrusio shell. The protrusio shell has one or two flanges for seating on the ilium, the ischium and the pubis. Accordingly, the medial thrust from the femoral head is dispersed through the flanges to the ilium, ischium and pubis.
One problem with hip joint prostheses is dislocation, i.e., removal of the femoral head from the acetabular cup. As described in my above-identified copending application, to help prevent dislocation, the acetabular cup may have an overhanging portion or extension at least in the superior-posterior region. Although a construction of this type is quite effective in reducing the likelihood of dislocation, it may not provide the desired range of motion.
As set forth in my copending application, it is desirable to implant the protrusio cup as a single unit in a one-step surgical technique. This, however, requires the preattachment of the protrusio shell and the acetabular cup in a predetermined orientation relative to each other. Previously attempted techniques for attaching the protrusio shell and the acetabular cup have created assembly problems that have resulted in damage to, and consequent scrapping of, some of the acetabular cups.